Light meter



' zza `Septn 1o, 1946. E. J. MARTIN ETA. l 2,407,564

. LIGHT METER Filed Margn s, 1944 i 515% 1 Cttorneg;

Patented Sept. 10, 1946 seres LIGHT METER Edward J. Martin, Pleasant Ridge, and carl E. Grinstead, Detroit, Mich., assignors to General Motors Corporation, Detroit,

. tion of Delaware Mich., a corpora- Application March 6, 1944, Serial No. 525,318

1 Claim. l

This invention relates: to measuringmeans and more specically to means for measuring minute quantities of light rays or the glow provided by phosphorescent material which has previously been activated bylsome activating means.

There are of course many instances in which it isv desired to know the strength of phosphorescence or ray emitting value. of a body or surface and as one example'of this may be cited means for'printing on sensitized paper from some phosphorescent source. There are in use processes utilizing a layer of phosphorescent paintv or radiating material applied to a` backing means which has imposedthereon certain configurations which it is desired to reduce, such asdrawings, prints, etc. One method of accomplishing this drawing reproduction'hasbeen to expose the phosphorescent material to a strong activating means which may be either X rays or light, depending upon the character of the phosphorescent means, and then removing, theysource of` activation and,` applying tothe membera sheetof sensitized material for printing.

However, the rat'eofdecay of glowor radiation of the phosphorescent means varies considerably and it is known that if the temperature is low, this decay in radiation strength can be slowed down considerably. However, `before applying the sensitized sheet to make a print or reproduction of the configuration on the phosphorescent means, it is desirable to know Whether or not there is suiiicient ray activity present in the phosphorescent means to provide printing and also as to the length of time it will be necessary to maintain the sensitized sheet in contact for eX- posure to obtain a satisfactory print. `It will be evident that the amounts of light or activated rays under consideration herein is small.`

It is therefore an object of our invention to provide sensitive means for accurately measuring small amounts of radiation in the light spectrum.

It is a further object of our invention to provide means for measuring rays in the light spectrum which is relatively small, portable, sensitive and rugged.` l

With these and other objects in view which will become apparent as the speciiication proceeds, the embodiments of our invention will be best understood by the following specication and claim and the illustrations in the accompanying drawing, in which the -gure is a circuit diagram showing the electrical system embodying our invention.

Referring now more specically to the gure,

(Cl. Z50-41.5)

2 there is shown therein a multiplier pho'totube 2 having a cathode 4 and a series of multiplier anodes 6, 8, Il?, |21 i4, |t,.l8, 2U, 22 and 24. This phototube is the means which is exposed to the i radiation to be measured and is mounted inside of a small box (not shown) having a Window `therein through which the rays may reach the head of the phototube. The cathode 4 is connected through line 25 tothe anode12'8- of a rectifier tube 30, the cathode 32` or filament of which is connected across a secondary winding'Sd of a lbeing connected by line 5BA to a` second terminal A of secondary 5u. These two' half Wave rectiers 30: and M `therefore provide across-the two' condensers 54, 5S a relatively stable voltage for `the various anodes of the photomultiplier tube 2.

In order, however, to have the extreme accuracy` necessary and to` provide the voltage applied to each'anode of the same proportion, it is necessary to provide additionalmeans for maintaining these individual voltages constant over uctuationsin theline voltage. The output voltage applied to lines 2B', 52 from the rectiiier is applied across a plurality of resistors 6e, S2', 64, B5, 68, is, l2, 1li, 15 and 18, al1 connected in series, which formons large resistance for the' total voltage. However, the various interconnecting lines feeding the anodes 6, 8, Hl, I2,` lli', It; 58, 2li, 22 andi@ are `all connected-tointermediate positions between these resistances, thus applyingto each anode aproportionate part oithe total voltage `across the output; In order to pro.-

' vide a. satisfactory voltagei'regulaton for each smallresistance; a voltage regulator tube', such as B8, is connected in parallel across each of the resistances El, 62, Bil, 5S, E8, 1i), 12, 14, 'l5 and lil. These tubes are well known in the art, are gas-filled and maintain the voltage across the same constant under varying supply. As an eX- example, the voltage tube used in the present instance has the commercial designation VR--30.

`As the electrons are bombarded Within the mulintermediate points of connection between these resistances are connected to taps and are capable of cooperation with an adjustable contact connected to line 94 which in turn is connected to a limiting resistance 96 andthence through line 98 to control electrode |00 of the `power amplier |02. Line 98 is also connected through condenser |04 to ground. -Thus tap 94 may be connected to any one of the series of taps provided between the resistances 86, 88, 90 and 92, depending upon the sensitivity available or desired.

A second source of power is provided for the power ampliers 02, |06 which form a portion of lthe bridge Vacuum tube voltmeter system provided to measure the output of the photoelectric tube 2. This power supply comprises a second multi-winding transformer |08 having a single primary and two secondary windings ||2 and I I4. Windingrl I4 provides a small voltage for the heater circuit ||6 of the tubes |02 and |06, and is shown as connected through by arrows to simplify the wiring diagram. Secondary I|2 -has a center tap connected through line ||8 with a bridge input connection between two resistors|20 and |22 which are connected to cathodes |24 and |26 of the power tubes |02 and |06, respectively, and also to a further grid |28 and |30 of the same tubes. The two ends of the secondary coill ||2 are connected through lines |32 and |34 to the electrodes |36 and |38 of a full-wave rectification tube |40.

An electrode |42 of the tube |40 is connected to resistors |44 and V|46 in series, which form a portion of a filter circuit in combination with capacity |48 and. with voltage regulator tube |50 which is similar to tubes 80, above described, provides a regulated output from Vthe Vfull-Wave rectifier to the bridge Vcircuit incorporating the vacuum -tube voltmeter for measuring means. Line |52 connects resistor |48 and one side of the voltage regulator |50 to a variable tap |54 on Vresistor |56. Resistor |56 has one terminal connected through a second resistor |58 to line |60 to form a third bridge arm which line is commonly connected to the "plate |62 of the power amplifier |02V and also to a third grid |64 of the same tube. The oppositek end of resistor |56 is in like manner connected through a third resistance |66 to connecting line |68 to complete the bridge, which line is jconnected to plate and the third grid |12 of the power amplifier |06. Connected across the bridge between lines |60 and |68 is a milliammeter |14 in series with a variable resistance |16.

In the operation of the system as disclosed, the

Vpower supply for transformer '36 provides alternating current which is rectified and developed as Vdirect current across the resistor formed of re- 4 Y sistances 60, 62, 64, 66, 68, 10, 12, 14, 16 and 18. There are then taken from this total voltage, voltage steps applied to the anodes 6, 8, I0, l2, I4, I6, I8, 20 and 22 which are carefully regulated so that each will be the same as the other and that they will be maintained as nearly constant as possible so that no errors will be introduced due to fluctuations in line voltage. Thus when light is directed on the photo-multiplier through radiation from a phosphorescent surface, an electron stream will flow from cathode 4 to anode 6;

This will dislodge additional electrons from anode 6 and cause the original stream tov be reflected on the anode 8. Thus a larger group will impinge on anode 8 and this will continue to multiply around'the tube to provide a useable amount of energy from a small excitation. Thus voltage will be developed in the line 82 proportional to the amount of excitation of the phosphorescent surface. Y

This voltage will be applied through one 'o the taps associated with one of the resistances 86, 88, 90 and 92 and to the control grid|00 of power amplifier-|02. This will change the relationship in the balanced bridge composed of power ampli'- ers |82, |06 and resistances |20 and |22 in two arms and resistances |66,` |56 and |58 which may be `balanced by the position of the tap V|54 on |56. At this time of course power is being supplied to the balanced bridge and speciflcallyrto the power ampliiiers |02 and |06 through the previously described circuit incorporating transformer |08. In this case also theoutput of the full-wave rectier is regulated by Voltage regulator tube to eliminate as much as possible any uctuaton in the power supplied to the bridge, and any variation in the supply to the amplifier tubes is applied equally to bothV and since they are situated in adjacent bridge arms, the effect will cancel out. Y v

Thus as the current flow through one leg of the bridge varies due to the output of tube 2, current will flow across the bridge through the milliammeter |14 vand this current will be directly prof portional to the amount of unbalance as introduced by flow through power amplifier |02. Thus milliammeter |14 may be calibrated to read di-V rectly in terms of radiation from the surface of the phosphorescent layer.

We claim:

In means for measuring the luminescent radiation of small quantity from a phosphorescent surface, a source of alternating current, means for rectifying said current connected Vto said source, tapped resistance means connected across the rectifier output, said'tapsV dividing said resistor into a plurality of substantially equal parts, a

, photosensitive multiplier tube having a plurality of electrodes sequentially engagedV by electron streams, means for connecting each of the electrodes to a tap andvoltage regulation means connected across each adjacent pair of taps topro-` vide constant voltage to each electrode.

EDWARD J.MARTD.\I. CARL E. GRINSTEAD. 

